John Wesley on Numbers 18:19

A covenant of salt - A durable and perpetual covenant; so called here and 2Ch 13:5, either, because salt is a sign of incorruption, as being of singular use to preserve things from corruption: or, because it is ratified on their part by salt, which is therefore called the salt of the covenant, for which the priests were obliged to take care, that it should never be lacking from any meat - offering, Lev 2:13. And this privilege conferred upon the priests is called a covenant because it is given them conditionally, upon condition of their service, and care about the worship of God.